1. Field of the Invention
The present invention relates to a mobile satellite communication system, and more particularly a mobile satellite communication system for performing communication between a mobile terminal and a base station apparatus by use of a communication system of utilizing a randomly access control system and a reserving system.
2. Description of the Related Art
A communication system which utilizes a randomly access control system and a reserving system described below is well known as a communication system in a mobile satellite communication system for performing a communication between a mobile terminal and a base station apparatus through a satellite.
In such a mobile satellite communication system, a plurality of mobile terminals perform a short burst transmission having a long guard bit pattern to the base station apparatus through a return link. The return link is a communication channel used for the transmission from the plurality of mobile terminals to the base station apparatus. For this reason, there may be a possibility that competition for the utilization of the return link occurs between the respective mobile terminals. Therefore, an access control signal AC is transmitted from the base station apparatus to the mobile terminals through a forward link so that the competition relation can be arbitrated. Such a forward link is a communication link used for a transmission from the base station apparatus to the respective mobile terminals.
FIG. 1 shows a structure of the access control signal AC. In FIG. 1, the access control signal AC has the field of a 1-bit I/B (Idle/Busy) flag, the field of a 16-bit partial echo signal (hereafter, to be referred to as a PE signal), the field of a 8-bit time alignment correction value TA, and the field of 8-bit CRC code. The I/B flag is set to "1" when the transmission using the return link is possible, i.e., the return link is in an idle state (I). On the other hand, the I/B flag is set to "0" when the transmission using the return link is impossible, i.e., when the return link is in a busy state (B).
The partial echo signal includes the information to specify one of the mobile terminals, from which reservation for a long burst transmission to be described later using the return link is transmitted to the base station apparatus.
The time alignment correction value TA is the information to specify a transmission timing of the long burst transmission.
The CRC code is a code added to carry out error detection and correction of the above-mentioned I/B flag, PE signal and time alignment correction value TA.
The above-mentioned access control signal AC is always transmitted from the base station apparatus to the respective mobile terminals through the forward link. Each mobile terminal always monitors the access control signal AC transmitted through this forward link. The mobile terminal performs a short burst transmission only when the I/B flag is set to "1" of the transmission permission state. At the same time, the mobile terminal starts a clocking operation of a timer.
Upon reception of this short burst transmission, the base station apparatus transmits to the mobile terminals, the access control signal AC including a response to the short burst transmission. Thus, the short burst transmission from the mobile terminal to the base station apparatus is ended. If the transmission from the mobile terminal to the base station apparatus is completed in the short burst transmission, the transmission from the mobile terminal to the base station apparatus is ended at this time.
When the I/B flag of the access control signal AC is set to "1", i.e., the transmission permission state, the short burst transmissions may be often performed by the plurality of mobile terminals at the same time. In this case, the receiving operation is performed in the base station apparatus, in a condition that short burst transmissions from the plurality of mobile terminals collide with each other. As a result, an error is detected in a CRC check operation to the received short burst transmissions. In such a case, the base station apparatus regards that the short burst transmissions are not received from any mobile terminal. At this time, the mobile terminal measures time by the timer after the short burst transmission. If this timer is set to the state of time-out without any reception of an access control signal including the PE signal from the base station apparatus after the short burst transmission, the mobile terminal determines that the short burst transmission fails, and again tries the short burst transmission.
The above-mentioned description gives the communication between the mobile terminal and the base station apparatus, when the data transmitted from the mobile terminal to the base station apparatus can be accommodated in the single short burst transmission.
On the contrary, there may be a case in which the transmission data from the mobile terminal to the base station apparatus is long so that the transmission data cannot be accommodated in the single short burst transmission. In this case, the base station apparatus makes a reservation such that the mobile terminal, which has performed the short burst transmission can use the return link for transmission of the subsequent data. Then, the base station apparatus transmits the access control signal AC including the time alignment correction value TA for specifying a transmission timing and the PE signal including a specification data identifying the mobile terminal corresponding to the reservation, to the mobile terminal through the forward link. Upon reception of this access control signal AC, the corresponding mobile terminal performs a long burst transmission including the whole or a part of the subsequent data through the return link, at the transmission timing specified based on the time alignment correction value TA.
The above-mentioned description gives the communication from the mobile terminal to the base station apparatus by use of the reserving system.
In the above-mentioned conventional mobile satellite communication system, the mobile terminal can know the failure of the short burst transmission at a time point when the timer started at the short burst transmission is set to the time-out state. That is, the mobile terminal can know the failure of the short burst transmission only after elapse of a predetermined time from the short burst transmission. Thus, the recognition as to whether or not the short burst transmission has been successful is different between the mobile terminal and the base station apparatus. As a result, there is a problem that there is a case that the communication is not correctly performed.
In addition to the above conventional mobile satellite communication system, a line setting request call originating system in a satellite communication system is described in Japanese Laid Open patent Application (JP-A-Showa 63-86621). In this reference, base stations are grouped into a plurality of groups. Each group includes two or more base stations. Also, one frame on a common channel, on which a line allocate request is transmitted, is composed of a plurality of integration time slots respectively corresponding to the plurality of groups. Each integration time slot is divided into a plurality of small time slots. A call originating operation is performed to the integration time slot in units of groups by use of a TDMA system. A call originating operation is performed to the small time slots of the integration time slot corresponding to one group in units of base stations of the group by use of a random access system with slot.
Also, a control signal sending system is described in Japanese Laid Open patent Application (JP-A-Showa 64-24522). In this reference, a control LSI (1) of a satellite communication TDMA apparatus has a burst counter (8) and performs the control of sending a control signal by performing a process in units of bursts. The control LSI also has a basic frame counter (6) for counting basic TDMA frames and a buffer (2) holding the control signal. The control LSI accesses an upper bit portion of the buffer (2) based on the output of the basic frame counter (6) and a lower bit portion of the buffer (2) based on the output of the burst counter (8). Thus, the control signal held in the buffer (2) is sent the number of times corresponding to the number of transponders for every basic frame.
Also, a satellite communication system is described in Japanese Laid Open patent Application (JP-A-Showa 64-24641). In this reference, the satellite communication system is composed of a main station and a plurality of child stations. A common carrier wave is used for transmissions from the child stations to the main station. In this system, a random access system is adopted in which each child stations sends a signal in units of slots obtained by partitioning the carrier wave on a time axis. A single carrier wave is used for transmission from the main station to the child stations. A signal is sent to each child station in a time divisional manner by use of a TDM channel. A frame on a random access channel is produced using a frame sync signal sent on the TDM channel and divided into a plurality of slots in order. When data are to be collected from n child stations, a control is performed such that specific ones of the n child stations send data on predetermined slots and such that remaining ones of the n child stations other than the specific child stations do not send data on the predetermined slots. This control is performed to different child stations in order in accordance with necessity.
Also, a satellite communication system is described in Japanese Laid Open patent Application (JP-A-Heisei 4-135330). In this reference, the satellite communication system is composed of a center station and a plurality of peripheral stations, each of which has a transmission section for accessing the center station through a satellite and a common channel. In this system, therefore, propagation times from the respective peripheral station to the satellite are different from each other. Each of the peripheral station is composed of a frame synchronizing section for extracting a reference timing signal from a transmission signal arriving from the center station, and a transmission timing control section for generating a transmission timing signal based on a time slot designating signal and for sending the transmission timing signal to the transmission section. The transmission timing signal is used to control a transmission timing of its transmission signal synchronous with the reference timing signal on the basis of a unit of a plurality of symbols.
Also, a satellite communication system is described in Japanese Laid Open patent Application (JP-A-Heisei 5-63671). In this reference, a plurality of peripheral stations is grouped into groups in unit of predetermined number of peripheral stations. A center station C allocates a time slot in units of groups. Mini-slots are provided for the time slot accessed by the peripheral stations. The mini-slot is fixedly allocated to each peripheral station. It is supposed to be two groups. Peripheral stations a, b and c belong to a group 1. The peripheral stations a and b transmit data Al and b1 on the third time slot of the first frame 1 and signals to the mini-slots allocated to them. The center station C monitors the mini-slots. The center station C determines that collision occurs, since the two mini-slots are used in the third time slot of the first frame 1. As a result, the center station C allocates retransmission slots to the peripheral stations a and b and a random access slot to the peripheral station c to the first, third and fifth time slots of the third frame 3. Thus, the retransmissions of the data A1' and B1' from the peripheral stations a and b is successful in a single cycle. Also, the peripheral station c can transmit data C1 without collision with the retransmitted data A1' and B1'.